This invention relates to an apparatus for interfacing a liquid chromatograph with a mass spectrometer. More particularly, the present invention is directed to the combined facilities of a liquid chromatograph and a high resolution double-focusing mass spectrometer. The method of the invention includes volatilizing a liquid chromatograph eluate, separation of the solvent liquid, and transport of high boiling substances of interest to the mass spectrometer in the form of suspended particles for analysis.
Mass spectrometric analysis of gas chromatograph fractions is known. It has been recognized that certain classes of organic substances while amenable to mass spectrometric analysis cannot be separated by passing through a gas chromatograph. Therefore, some other means of separating such materials is required as preparation for mass spectrum studies and identification.
A common method for separating such materials is by liquid chromatography. Liquid chromatography is a technique which is useful in analyzing substances which comprise large or polar molecules and therefore unsuitable for gas chromatography.
Liquid chromatography provides a means for separating complex mixtures of either organic or inorganic mixtures into their various components, for example, compounds that are thermally unstable or nonvolatile under normal gas chromatographic conditions.
Another widely used technique for providing structural information about chemical species is mass spectrometry. Mass spectrometry identifies unknown species by comparison of its mass spectrum with a reference mass spectrum obtained from a known composition. Mass spectrometry generally requires that the mass spectra be generated by the electron impact mode of ionization and mass spectra should be limited to only one species at a time
In liquid chromatography, a chromatographic solvent containing a mixture of components in solution, is passed through a chromatographic column. The chromatographic column is designed so that it separates the mixture, by differential retention on the column, into its various components. The component species emerge from the column as distinct bands in the solvent stream separated in time and therefore distinguishable by the relative retention times. Thus, a liquid chromatograph provides a means for introducing single species of substances which have been separated from an initially complex mixture into a mass spectrometer.
While liquid chromatography provides a means for separating a mixture into its components, in order for an accurate mass spectrum of the various components to be obtained, the chromatographic solvent remaining in the eluent must be separated and removed from the material delivered to the mass spectrometer. Without removal of the solvent from the material entering the ionization chamber of the mass spectrometer, the mass spectra obtained cannot be used for precise identification of the compounds or materials.
The organic liquids that are used as eluents in liquid chromatography, if present even in minute amounts, constitute a major source of error in any subsequent mass spectrometric analysis. This occurs because the material entering the mass spectrometer exits the liquid chromatograph in the form of a solution which generally has a concentration in the range of 10-100 parts per million (ppm) of eluent. As a result, an interfacing means and method which removes the solvent while efficiently transferring the material of interest to the mass spectrometer is needed and is provided by this invention.
One approach to the removal of solvent from a sample prior to mass spectrometry is disclosed in U.S. Pat. No. 4,629,478 which shows a constant flow of solvent solution containing the material to be analyzed to a monodisperse generator. The monodisperse generator generates droplets of particles of both solvent and solute which have a narrow range of sizes. The finely dispersed solvent aerosol is diluted with a gas, usually an inert gas such as argon or helium, and passes into a low pressure desolvation chamber. In the desolvation chamber, the majority of the solvent evaporates and is separated from the solute, i.e., the material to be analyzed by the mass spectrometer. The combination of dispersion gas and solvent vapor then passes sequentially through two pressure reduction chambers where the dispersion gas and solvent vapor is removed by vacuum pumps. A highly dispersed aerosol of sample material remains after solvent evaporation in the vacuum chambers. This aerosol enters the ionization chamber of the mass spectrometer where ions are generated for subsequent mass analysis.
Another design for an interface utilizing particle beam technology is currently marketed by Hewlett-Packard Company. This device uses a pneumatic nebulizer and a two-stage momentum separator. The stated sensitivity specification for the Hewlett-Packard device is a signal/noise ration of 50:1 on the molecular ion of caffeine using a sample size of 20.times.10.sup.-9 g. and an LC flow of 0.5 ml/min. methanol.
Other techniques for combining liquid chromatographs and mass spectrometers are disclosed in Winkler et al., 60 Anal. Chem. 489-93 (1988) and Willoughby et al., 56 Anal. Chem. 2626-2631 (1984). In Winkler the effluent from the liquid chromatograph column is pumped through a small diameter orifice thereby forming a liquid jet. The liquid jet spontaneously breaks through Rayleigh interactions with the surrounding gas. The stream is subjected to a perpendicular flow of helium gas which disperses the particles. The primary drop stream is then subjected to a secondary shearing force which reduces drop size. Upon travelling to an unheated desolvation chamber the stream passes to a two stage momentum separator.
Willoughby discloses a two-stage aerosol-beam separator in conjunction with a monodisperse aerosol generator and a desolvation chamber. Heat is applied to the desolvation chamber by means of a heating tape wrapped around the chamber.